Pulsewidth modulation (PWM) may be used to control various electrical circuits and systems. For example, PWM may be used to control DC-DC converters, electric motors, lighting systems, audio amplifiers, etc. Embodiments of PWM controllers will be described below in the context of DC-DC power supplies, but the PWM controller embodiments may be used to control various other systems as well.
PWM control of a DC-DC power supply requires very precise adjustment of PWM ON/OFF periods to achieve the required ripple on the output DC rails. Previous solutions could only achieve this level of precision in the analog domain.
FIG. 1 illustrates a simple DC-DC power supply according to the prior art. The DC-DC power supply 100 includes an operational amplifier 110, a comparator 120, a ramp generator 130, and a boost converter 140. The boost converter includes an inductor 142, switching transistor 144, a diode 146, and a capacitor 148. The input voltage Vin applied to the inductor 142 which is connected to the transistor switch 144 which opens and closes with a specified duty cycle. The result is an output voltage Vout that is proportional to Vin according to the duty cycle. Accordingly, as the duty cycle of the switch 140 varies, different values for Vout may be obtained. The boost converter is but one of many different switched designs, where a PWM input signal drives an output voltage that depends upon the duty cycle of the PWM input signal. The operational amplifier 110, the comparator 120, and the ramp generator 130 act as a pulsewidth modulator.
The DC-DC power supply operates by feeding back a sample of the output voltage and subtracting this voltage from a reference voltage to establish a small error signal (VERROR) using the operational amplifier 110. This error signal is compared to an oscillator ramp signal using the comparator 120. second comparator 120 outputs a digital output (PWM) that operates the power switch. When the circuit output voltage changes, VERROR also changes and thus causes the comparator threshold to change. Consequently, the output pulse width also changes. This duty cycle change then moves the output voltage to reduce the error signal to zero, thus completing the control loop.
This method allows analog control that achieves very fine granularity adjustments to the PWM signal. Other prior art systems use delay lines to achieve required PWM granularity.
These and other prior solutions cannot support scaling to a very large number of PWM controllers. Further, the prior art requires special circuits to control the PWM. This limits the number of instances that can be realized in a given design.